This invention relates to improvements in elastomer-based friction materials and more particularly to such friction elements, i.e., clutch, brake in the like, which run in oil and are referred to as wet friction materials. In a typical application, the friction element is secured to a backing plate, and the element is brought into engagement with a relatively moving reaction plate to achieve power transfer or braking action.
Prior art wet friction materials have traditionally included friction elements made from paper, or from sintered bronze, or from a composite having a resin binder such as phenol formaldehyde. Although these materials are useful over a wide range of applications, the advent of larger and more powerful industrial vehicles has created a need for a material having even higher energy absorption rates and an improved service life at higher than normal operating temperatures.
More recently, elastomers have been used as the binder or base material in friction materials, as for example are described in U.S. Pat. Nos. 3,898,361, 4,042,085, 4,045,402 and 4,131,590. The use of an elastomer has several advantages, inasmuch as the resultant material has greater elasticity than traditional materials and therefore exhibits better energy absorption rates. The relatively low modulus of the material also allows the friction to better conform to irregular or uneven surfaces of the mating member without undue wear.
A primary limiting factor in the use of elastomers in friction materials is the thermal stability of the elastomer. In the event the thermal stability of the elastomer is exceeded, the friction element will lose its strength and fail rapidly, thus requiring replacement. For this reason, it would be desirable to select an elastomer which has a high thermal stability.
Of the elastomers that are presently available on a commercial basis, fluoroelastomers exhibit superior thermal stability. U.S. Pat. No. 3,898,361 in particular describes a friction material composed of a fluoroelastomer matrix into which is incorporated silica-based or glass fibers. The glass fibers serve to reinforce the elastomer and also serve as friction fillers to provide the desired coefficients of friction.
Although friction materials based on mixtures of fluoroelastomers and silica-based fibers offer excellent performance, the industry continues to seek even better performance, particularly at higher energies and temperatures experienced in advanced equipment. Especially at higher temperatures, it has been found that the fluoroelastomer and glass fiber composite tends to become brittle and weak causing the material to fail prematurely.